The present invention relates to a control circuit for an electrically adjustable patient chair and in particular to a circuit that is adapted when activated to automatically return the chair to its normal tilt-forward and lift-down position.
Patient chairs utilized in medical and dental offices typically have electrical height and tilt controls. It is therefore desirable to provide a control circuit for automatically returning the chair to its exit position without having to hold the lift-down and tilt-forward return switches. Several approaches to this problem are possible.
The simplist method, of course, is simply to install a maintain double-pole switch in parallel with the lift-down and tilt-forward return switches which when closed will energize the height and tilt adjustment motors until the motor limit switches deenergize the respective motor windings. However, with this approach the user must remember to reset the maintain switch to its off position before the height and tilt controls can again be utilized.
Another approach which automatically performs the reset function involves the use of a double-pole relay in place of the maintain switch and a timer that is adapted to time-out and unlatch the relay after a period of time sufficiently long to permit the chair to return to the exit position from its extreme up and back positions. However, with this approach, the normal height and tilt control switches are inoperable for a given period of time, regardless of how close the chair is the exit position when the automatic return is activated. In addition, timer circuits capable of delay times of sufficient duration for this purpose can be quite expensive.
The present invention however proposes to solve these problems by providing a logic circuit that senses the current to the windings of the height and tilt adjustment motors and unlatches the relay when the current falls to zero. More particularly, the present control circuit utilizes a current transformer that senses the current through the motor windings and removes the bias signal to the gate of a solid state switching device when the current through the windings goes to zero. The solid state switching device, which is connected in series with the relay coil, is thereby turned off, deenergizing the relay coil and opening the double-pole relay contacts connected in series with the motor windings. In this manner, the control circuit is automatically reset when the chair attains its exit position.
Thus, it will be seen that a highly simplified, inexpensive logic circuit is provided that permits a patient chair having electrical height and tilt controls to be automatically returned to its exit position simply by momentarily depressing a single switch. In addition, the present control circuit is designed to be easily incorporated into the electrical system of the chair without having to significantly modify the existing wiring or adding additional leads from the height and tilt adjustment motors.